With the rapid growth of the information industry and continuous breakthroughs in the display technologies, a trend is becoming increasingly recognizable in that flat panel displays (FPD), which take up a much smaller space, are gradually taking the place of the traditional cathode ray tubes (CRT). Among the various flat panel displays, liquid crystal displays (LCD) have assumed a leading position, because of their light weight, small thickness, low driving voltage required, and low energy consumption. Another reason for the wide popularity of LCDs can also be attributed, at least in part, to the rapid development of the technologies that LCDs have been associated with. More recently, with the successful development of thin film transistors (TFT), LCDs now have acquired the capability of becoming a full-color display ready for a much broader consumer market. This development further enhances the already immense potential of LCDs.
With both the multi-color and full-color LCDs, chroma control and brightness control are the two most essential elements. These elements are provided by a high-grey-level (black-and-white) LCD, color filter films, and backlight devices. Of these, color filters provide the most important role for color control.
A color filter comprises three main components: a black-hued (i.e., black-colored) matrix, a color filter layer, and an overcoat. Currently, at least five methods have been disclosed in the prior art for making color filter layers. These include:
(1) dyeing, PA1 (2) etching, PA1 (3) pigment dispersion, PA1 (4) electrodeposition, and PA1 (5) printing. PA1 (1) Forming an energy-accumulable positive photoresist layer on a transparent electrically conductive substrate, and exposing the photoresist layer to a light source so as to form three or four regions of different exposures having exposure energies of D.sub.1, D.sub.2, D.sub.3, (and D.sub.4), respectively, where D.sub.1 &gt;D.sub.2 &gt;D.sub.3 (&gt;D.sub.4), and D.sub.1 represents the full exposure energy (i.e., energy required for full exposure in order to facilitate subsequent development). This step is called "pro-conditioning" of the photoresist, to form a plurality of regions of initially different exposure energies. PA1 (2) Using an appropriate developer solution to remove the region of the photoresist layer with an initial exposure energy of D.sub.1. Only this portion of the photoresist layer is removed, and, as a result, the corresponding area of the conductive substrate (i.e., underlying the D.sub.1 region) is uncovered. This exposed area is electrodeposited with a photo-curable resin of desired color hue. This step is termed as n=1. PA1 (3) Exposing the entire surface of the photoresist layer to an incremental exposure energy of IE.sub.n, IE.sub.n, (=D.sub.n -D.sub.n+1) is the gap, i.e., difference, between two adjacent exposure energies. This step increases the exposure energy received (through accumulation) by the entire photoresist layer by an incremental amount of IE.sub.n. Thus, the region of the photoresist layer originally having an exposure energy of D.sub.2 is now elevated to an exposure energy D.sub.1 (D.sub.1 =D.sub.2 +IE.sub.1). This allows the originally D.sub.2 area (in step 1) to be developed by the same developer solution, and the newly uncovered area on the conductive substrate is electrodeposited with a photo-curable resin of another color. This step is described as n&gt;1. This incremental exposure energy also causes the photo-curable resin to become hardened. These are the two important elements of the present invention. PA1 (4) Repeating step 3 until all the thermosetting resins of desired colors are electrodeposited, respectively, on predetermined regions of the conductive substrate.
The dyeing method and the etching method primarily utilize an appropriate arrangement of dyes to prepare color filters. A wide variety of dyes have been taught in the prior art references many of which provide homogeneous chroma, high dyeability, and allow a wide selection of compatible resins for which desired color intensity and light transmissibility can be provided. U.S. Pat. No. 4,820,619, the content thereof is incorporated herein by reference, a photosensitive composition is disclosed for use in preparing a color filter which contains a copolymer of glycidyl (meth)acrylate or glycidyl (.alpha.-methyl)vinyl ether with a (meth)acrylic amide or ester having a quaternary ammonium salt structure, and an aromatic azide as a photosensitizer. U.S. Pat. No. 4,837,098, the content thereof is incorporated herein by reference, discloses a colored filter layer comprises three groups of filter picture elements having spectral characteristics respectively corresponding to red, green and blue. Each group of filter picture elements (R, G, B) are made of polyimide resin and dye contained therein.
Because of the relatively inadequate light and heat resistances of the dyeing materials, the methods of dyeing and etching discussed above have been largely replaced by the pigment dispersion method and/or the electrodeposition method, both of which utilize pigments that exhibit superior light and heat resistances. In these methods, pigment particles are uniformly dispersed in a resin matrix. Typically, the pigment particles are controlled to have a particle size less than 0.2 .mu.m so as to ensure reliable coloring characteristics. U.S. Pat. No. 5,085,973, the content thereof is incorporated herein by reference, discloses a color filter prepared by providing red, green, and blue image elements, each imaging element comprising a photosensitive resin and a pigment, and a black matrix on a transparent glass substrate. The photosensitive resin is formulated such that it comprises a polyfunctional acrylate monomer, an organic polymer binder and a photopolymerization initiator comprising a 2-mercapto-5-substituted thiadiazole compound, a phenyl ketone compound, and 2,4,5-triphenylimidazolyl dimer composed of two lophine residues combined to each other through intermediation of a single covalent bond. U.S. Pat. No. 4,786,148, the content thereof is incorporated herein by reference, discloses a color filter comprises a substrate and colored resin films, including blue, green and red resin films containing blue, green, red colorant particles, respectively. The average particle volumes of the blue, green and red colorants are set that the blue particles are greater than the green particles, which are further greater than the red particles. The pigment method is also disclosed in, for example, Japan Laid-Open Patent Publication JP60-129739. With the pigment dispersion method, lithographic techniques can be utilized to improve resolution, increase the flexibility of pattern design, and form color filters that can be used in TFT-LCDs. However, the conventional pigment-related methods typically involve a relatively complex process, and they require at least three photomasks which must be precisely aligned to ensure good quality. Furthermore, because the pigment dispersion method involves a free radical reaction to form a thermoset resin, a protective layer is required so as to avoid contact with oxygen.
U.S. Pat. No. 4,812,387, the content thereof is incorporated herein by reference, described an example of the electrodeposition coating process, by which a coating film is formed for filling the space between the color stripes for a color filter which is used for the colorization of a liquid crystal display. With the electrodeposition coating processes, a transparent electrode is prepared by patterning a transparent electrically conductive film (typically an indium-tin oxide, or ITO) which is deposited on a substrate and serves as an electrode, and an electric voltage is applied only to a portion of the patterned transparent electrode which is to be dyed in the same color. The substrate is then immersed in a coloring electrodeposition bath containing appropriate polymers and pigment dispersed in water, and a colored layer is formed by electrodeposition. Thereafter, electric voltage is applied only to another portion of the substrate which is to be dyed in a different color, and the substrate is then immersed in another colored electrodeposition bath for forming a different color layer via electrodeposition. This procedure is repeated until all the desired colored layers are formed. The disadvantages of the electrodeposition coating process are that it is necessary to perform a high precision patterning of the transparent electrode, and to pay meticulous attention during the subsequent process not to break the fine pattern, because otherwise, the subsequent coloring process will be rendered very difficult. The electrodeposition coating technique is limited in its applications because it requires a substrate with a stripe pattern of conductive (ITO) film (the stripe pattern consists of a plurality of segregated parallel lines) for implementation, and it typically cannot be used without the stripe patterns. Thus, the electrodeposition coating processes are suitable for the preparation of color filters for use in STN-LCDs, and have limited applications.
Among all the processes for preparing color filters, the printing process is the least expensive process. However, it suffers the problems of poor dimensional precision, smoothness, and reliability, and is not well accepted by the industry for making high quality electronic products.
Nippon Oil Company proposed an electrodeposition lithographic method (ED-litho) for making color filters which combined the electrodeposition (ED) coating method and the lithographic (litho) technique. In U.S. Pat. No. 5,214,542, the content thereof is incorporated herein by reference, Nippon Oil disclosed an electrodeposition lithographic method, which involves the steps of: (a) forming a photosensitive coating film on a transparent electrically conductive layer provided on an outermost surface of a substrate having an alignment film, (b) exposing the photosensitive coating film to light through a mask having patterns of at least three different degrees of light transmittances; (c) developing and removing a photosensitive coating film portion registering with one of the patterns of smallest and largest degrees of light transmittances to expose the transparent electrically conductive layer; (d) electrodepositing a colored coating on the exposed electrically conductive layer to form a colored layer thereon, and (e) repeating the step (d) for the respective patterns of different degrees of light transmittances in sequence of difference in light transmittances to form different colored layers, respectively. U.S. Pat. No. 5,214,541, the content thereof is incorporated herein by reference, discloses the additional step of transcribing the colored layers, the transparent electrically conductive layer, and the alignment film onto another substrate.
The electrodeposition lithographic method discussed above has several advantages in that: (1) high precision patterns can be obtained, better than that obtainable from the electrodeposition coating method; (2) the pattern figure has a high degree of freedom, and both stripe and non-stripe patterns can be provided; (3) because it utilizes the advantageous characteristics of electrodeposition process, the coated films exhibit uniform film thickness and excellent smoothness. However, the electrodeposition lithographic method requires developer solutions at least three different levels of concentrations so as to selectively remove the exposed photoresist at different stages of the development process, thus it allows only a relatively narrow process window within which tolerance is acceptable, and there exist only very limited options in selecting an appropriate photoresist. Additionally, only a limited number of options of photoresist-developer combinations can be utilized. This is especially true when a positive photoresist is used, in which only cationic electrodeposition resins can be used and anionic cannot be used.
If an anionic electrodeposition resin is used, then the basic developer solution can easily remove those acidic colored resins that have been electrodeposited but have not been hardened. This problem is further complicated by the fact that the colored resin cannot be hardened by light or heat during the electrodeposition lithographic process so as to ameliorate the problem caused by the uncured colored resin. Therefore, when the electrodeposition lithographic method is used, a positive photoresist must be used in conjunction with a cationic electrodeposition resin. Only negative photoresists can be used with an anionic electrodeposition resin; however, it is well known that negative photoresists do not provide the same dimensional precision as positive photoresists, and the trend in the industry is using positive photoresists. It is also well-known to those skilled in the art pertaining to pigment chemistry that cationic colored electrodeposition resins exhibit far superior characteristics, including stability (against decolorization), ease of emulsification, pigment dispersability (especially at high pigment concentrations), as well as lower raw material cost than their anionic counterparts. Thus the electrodeposition lithographic method disclosed in the prior art, which, almost by default, requires the combination of positive photoresist and anionic colored electrodeposition resin, does not represent the best, or the most desired, choice.